The present invention relates to an open-magnetic-circuit type engine-igniting coil device.
Japanese Laid-Open Patent No. 60-107813 discloses an open-magnetic-circuit-type engine igniting coil device which has a coil case which contains an assembly integrally molded therein by potting with melted insulating resin and consisting of a primary coil bobbin having a hollow center with a rod-shaped core and a secondary coil bobbin coaxially positioned on the primary coil bobbin and which is further provided at its lower portion with a high-voltage terminal connector to be directly connected with a tip of an ignition plug of the engine.
In the conventional engine igniting coil device, the primary coil bobbin can be coaxially mounted into a hollow shaft or center of the secondary bobbin by abutting at its flange against the inside wall of the hollow center of the secondary bobbin. The coil assembly can also be coaxially mounted in the coil case by abutting at its flange against the inner wall of the coil case.
In the conventional engine igniting coil device of the open-magnetic-circuit type, the rod-like core of the primary coil bobbin is provided at each end with a permanent magnet for obtaining a large change in magnetic flux with an interrupted primary current.
Japanese Utility Model Publication No. 4-23296 also discloses an open-magnetic-circuit-type engine igniting coil device which has a coil case containing a coil assembly integrally molded therein by potting with melted insulating resin and consisting of a primary coil bobbin having a hollow center with a rod-shaped core and a secondary coil bobbin coaxially fitted on the primary coil bobbin and which is further provided with an ignition-plug connector portion integrally formed on the coil case for direct connection with a ingition plug in such a way that a tip of the ignition plug inserted therein can contact with a high-voltage terminal inwardly projecting in the connector portion of the coil case.
In this engine igniting coil device, there is used a laminated core 9 that is, as shown in FIGS. 8 to 10, a lamination of sheet materials 91 fixed by caulking or staking (e.g., V-shape, circular or pin caulking) or welding by fusing heat. FIG. 8 illustrates a laminated core with a V-shape caulked portion 92 and FIG. 9 illustrates a laminated core with a round caulked portion 93. FIG. 10 shows a laminated core with a welded seam 94.
The above-mentioned prior arts devices, however, involve the following problems to be solved:
The first problem is that an engine igniting coil device directly attached to an ignition plug has a long case to be inserted into a cylindrical bore in a cylinder head of a vehicle engine and said case may therefore have a large amount of thermal elongation and shrinkage of metal, producing a large axial stress in the insulation resin layer formed therein and, in consequence of this, causing cracking of the flanges of the primary and secondary coil bobbins.
The arrangement of the flange of the secondary coil bobbin close to a portion with the high-voltage-side terminal of the secondary coil may cause a leak current to flow through the flange to the coil case.
The second problem is that the conventional open-magnetic-circuit type engine igniting coil device has two permanent magnets 10 attached to respective ends of the rod-shaped core 9 with a side step formed therebetween as shown in FIG. 5: each stepped portion may serve as a start point of cracking C1 in the insulating resin layer around thereof by transmitting a thermal stress, resulting in a breakage of the secondary coil bobbin 8.
In the open-magnetic-circuit type engine igniting coil device, the rod-shaped core inserted in the hollow center of the primary bobbin may suffer a relatively large thermal stress produced in its longitudinal direction, causing cracking of the insulating resin layer enclosing the core.
Furthermore, the device usually uses a secondary coil bobbin formed by using a through-type molding tool to minimize uneven thickness of its wall because it is impossible to form a long slender type secondary coil bobbin with a specified even wall thickness. However, the secondary coil bobbin 8 formed by using the through-type molding tool has a hole 111 formed therein by a center pin of the molding tool as shown in FIG. 5. This shortens a creeping discharge distance between a high-voltage terminal 12 and a core 9 to reduce the durability of the coil device. The secondary coil bobbin 8 may suffer cracking C2 due to a stress produced by a differential shrinkage of materials of the bobbin 8 and the core 9.
The third problem of the conventional open-magnetic-circuit type engine igniting coil device has the rod-like laminated core inserted in a hollow center of the coil assembly with primary and secondary coils, wherein a magnetic flux produced therein diverges outwardly and may suffer a loss of a flux part passing through a cylinder block of the engine, resulting in a decrease of the output factor of the secondary coil. It is needed to provide additional means for preventing the loss of magnetic flux.
In this case, the conventional laminated core has a portion partially deformed by caulking (e.g., V-shape, round or pin cauking) or welding, where a loss of magnetic flux may arise.